A process of manufacturing rubber articles or plastic articles includes a process of mixing a plurality of kinds of materials such as rubber or plastic materials into a mixing object. A temperature of the mixing object is used as an index representing a degree of mixing of the materials. In view of the above, the temperature of the mixing object is measured during a mixing operation.
A thermocouple is used for measuring a temperature of a mixing object. The mixing object has a high viscosity. Therefore, contact of the mixing object being mixed with a thermocouple may damage the thermocouple. In view of the above, a protection tube thermocouple having a structure, in which a thermocouple is accommodated in a tubular member is used.
The protection tube thermocouple is a thermocouple having a structure, in which thermocouple wires are accommodated in a protection tube made of metal or ceramic. Examples of the aforementioned thermocouple are a protection tube thermocouple and a sheath thermocouple.
The protection tube thermocouple is deteriorated, as it is used. Therefore, a technique of detecting deterioration of the protection tube thermocouple has been proposed. For instance, as a technique of detecting deterioration of a protection tube thermocouple, the thermocouple deterioration detection device disclosed in patent literature 1 is proposed. The thermocouple deterioration detection device includes a first thermocouple whose tip portion is mounted on a measurement object; a second thermocouple which is provided in proximity to the first thermocouple and whose length is shorter than the length of the first thermocouple; and processing means which compares between the output of the first thermocouple and the output of the second thermocouple, judges that the first thermocouple is deteriorated when the output difference between the first thermocouple and the second thermocouple exceeds a predetermined value, and generates a deterioration detection signal.
A protection tube thermocouple is a very important element in measuring a temperature of a mixing object being mixed. However, due to the poor responsiveness of the protection tube thermocouple, a temperature lower than the actual temperature (e.g. a temperature lower by about 10 to 20° C.) may be displayed. As a method for improving the temperature responsiveness, there is proposed a method of reducing the volume of the protection tube in order to reduce the heat capacity.
However, reducing the volume of the protection tube may lower the strength of the protection tube. Therefore, there is a problem such that the protection tube may be broken during a mixing operation, and a broken part may be included in the mixing object. This may adversely affect the process. Further, patent literature 1 fails to disclose the description relating to improvement of the strength and responsiveness of the protection tube. There is room for improvement in this point.